1. Field of the Invention
The present invention relates generally to startup of polyolefin production with multiple polymerization reactors.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into everyday items. Polyolefin polymers such as polyethylene, polypropylene, and their copolymers, are used for piping, retail and pharmaceutical packaging, food and beverage packaging, plastic bags, toys, carpeting, various industrial products, automobile components, appliances and other household items, and so forth.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements, such as thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown, or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. These processes may be performed at or near petrochemical facilities, “‘which provide ready access to the short-chain olefin molecules (monomers and comonomers), such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor. As polymer chains develop during polymerization in the reactor, solid particles known as “fluff” or “flake” or “powder” are produced in the reactor.
The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), comonomer content, molecular weight, crystallinity, and so on. Different properties for the fluff may be desirable depending on the application to which the polyolefin fluff or subsequently pelletized polyolefin is to be applied. Selection and control of the reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, catalyst type, and so forth, may affect the fluff properties.
In addition to the one or more olefin monomers, a catalyst (e.g., Ziegler-Natta, metallocene, chromium-based, post-metallocene, nickel, etc.) for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may•be a particle added via a reactor feed stream and, once added, suspended in the fluid medium within the reactor. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction. Moreover, an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, n-hexane, and/or heptane, and so on, may be added to the reactor and utilized as a diluent to carry the contents of the reactor. However, some polymerization processes may not employ monomer as the diluent, such as in the case of selected examples of polypropylene production where the propylene monomer itself acts as the diluent. Nevertheless, the diluent may mix with fluff and other components in the reactor to form a polymer slurry. In general, the diluent may facilitate circulation of the polymer slurry in the reactor, heat removal from the polymer slurry in the reactor, and so on.
The slurry discharge of the reactor typically includes the polymer fluff as well as non-polymer components such as unreacted olefin monomer (and comonomer), diluent, and so forth. This discharge stream is generally processed, such as by a diluent/monomer recovery system (e.g. flash vessel or separator vessel, purge column, etc.) to separate the non-polymer components from the polymer fluff. The recovered diluent, unreacted monomer, and other non-polymer components from the recovery system may be treated and recycled to the reactor, for example. As for the recovered polymer (solids), the polymer may be treated to deactivate residual catalyst, remove entrained or dissolved hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, and so forth, before the polymer is sent to the customer.
In some circumstances, to increase capacity of a polyolefin polymerization line or to achieve certain desired polymer characteristics, more than one polymerization reactor may be employed, with each reactor having its own set of conditions. In certain examples, the reactors (e.g., loop reactors) may be connected in series, such that the polymer slurry from one reactor may be transferred to a subsequent reactor, and so forth, until a polyolefin polymer is produced discharging from the final or terminal reactor with the desired set of characteristics. The respective reactor conditions including the polymerization recipe can be set and maintained such that the polyolefin (e.g., polyethylene, polypropylene) polymer product is monomodal, bimodal, or multimodal, and having polyolefin portions of different densities, and so on.
The polymerization in a single or multiple reactors is generally exothermic, or heat-generating, and is typically performed in closed systems where temperature and pressure can be regulated to control production. As with any such closed system where heat is generated, some means should be supplied to remove heat and thus to control the polymerization temperature. For loop reactors and other polymerization reactors, a cooling or coolant system is typically used to remove heat.
During startup of reactors, however, heat is added to the reactor contents to facilitate initiation of the polymerization. The coolant system may include one or more heaters, which may be used to add heat to the contents of either reactor until the polymerization becomes exothermic. The heater may be, for example, a shell and tube heat exchanger, a mixer, or an educator, such as a pick heater.
After startup of the reactors, and when the reaction in the reactors becomes exothermic, the normal operation of the coolant system is to remove heat from the reactors.
Variations in reactor feedstocks, utility supplies, and reaction kinetics induce variations in the reactor (polymerization) temperature which may be mitigated by the reactor temperature control scheme and the reactor coolant system. The control scheme and coolant system should also accommodate reactor upsets caused, for example, by undesirable slug feed of reactants or by rapidly changing heat transfer behavior in a fouling reactor.
Unfortunately, problems may be experienced that cause the coolant system to remove too little heat or too much heat from the reactor. Poor temperature control in the reactor increases the cost to manufacture polyolefin. In particular, poor temperature control in the reactor results in a wider design basis for coolant system equipment and thus increases equipment costs. Furthermore, swings in reactor temperature impact reactor stability and can lead to a reactor foul and/or unplanned shutdown. Additionally, polymerization temperature affects the properties of the polyolefin and thus poor control of reactor temperature causes off-spec production of polyolefin. Moreover, employment of multiple polymerization reactors in a polyolefin reactor system may add complexity and cost of the coolant system and reactor temperature control.
The competitive business of polyolefin production drives manufacturers in the continuous improvement of their processes in order to improve operability and product quality, lower production costs, and so on. In an industry where billions of pounds of polyolefins are produced per year, small incremental improvements, such as in reducing capital and operating costs associated with reactor cooling while maintaining effective temperature control and product quality, can result in a more attractive technology and economic benefit including greater price margins and netback, and so forth.